Seismic surveying of the earth has been done for a number of years, particularly in the field of petroleum and gas exploration. The original seismic souce was an explosive, such as dynamite or TNT, which was detonated in a bore hole. The compressional seismic waves would be reflected or refracted by geologic structures within the earth. An array of sensitive geophones located at predetermined positions on the surface of the earth at various distances from the bore hole were responsive to the reflected and refracted waves, and the resultant electrical signals from those geophones were recorded and interpreted to yield much useful information.
There are many drawbacks in the use of explosives, including safety, cost of drilling bore holes and length of time needed to drill them, cost of explosives themselves and lack of repeatability. If a second and equal-size charge of explosives is detonated in the same bore hole, the resultant seismic waves will be different from the original waves, because the bore hole was distorted by the first explosion. Accordingly, explosives have, in many cases, been supplanted by alternate energy surface sources, such as those employing the "air gun". An air gun suddenly releases a charge of pressurized gas, usually compressed air, with powerful explosive-like abruptness, and this discharge can be predictably repeated time after time after time. Air guns have been incorporated in land seismic energy surface sources, examples being those disclosed and claimed in the following U.S. patents:
U.S. Pat. No. 3,310,128 PA0 U.S. Pat. No. 3,779,335 PA0 U.S. Pat. No. 3,800,907 PA0 U.S. Pat. No. 4,108,271 and PA0 U.S. Pat. No. 4,316,521.
These land seismic energy surface sources have included a tank, which in the last four of these patents is in the shape of an inverted dome. The tank is provided with a displaceable diaphragm bottom to contain an incompressible liquid such as water. One or more air guns are positioned within the tank and receive and store pressurized air, or other suitable gas, under high pressure supplied from a source such as the receiver chamber or pressure tank of a high pressure air compressor. The last three of these patents show a metal pan positioned between the diaphragm and the earth so that, upon the abrupt release of the high pressure gas from the air gun into the liquid, the flexible diaphragm suddenly thrusts down on the metal pan to suddenly thrust down on the portion of the earth beneath the pan and thereby produce a powerful seismic impulse. One of the aforementioned patents (U.S. Pat. No. 4,316,521) discloses the concept of inclining the entire generating assembly away from the vertical in order to accommodate local variations in the slope of the earth's surface. In each of the land seismic sources shown in these patents the seismic energy which is transmitted into the earth mainly is in the form of compressional (P) waves.
Geophysicists would like to produce shear (SH) waves in the earth in order to obtain considerable valuable additional information beyond that which can be obtained from compressional (P) waves alone. While some of the seismic energy transmitted into the earth by the above land surface sources is incidentally in the form of vertically polarized shear (SH) waves generated in the earth near the perimeter of the metal pan caused by sudden downward compressive thrust of the pan on the earth beneath the pan relative to the surrounding uncompressed earth, such incidental vertically polarized shear (SV) waves do not provide much significant useful information beyond that resulting from the powerful compressional (P) waves themselves.
One technique which has been employed in the past to obtain horizontally polarized shear (SH) waves has been to find or construct a relatively steep bank of earth. The seismic source is then tilted so as to direct its energy into the slanted surface of the bank, whereby the thrust is applied in a direction more nearly parallel with the earth's surface for generating SH waves. It will be understood that such a method of constructing steep earth banks is expensive in man-hours and also requires a great deal of work to restore the surface of the earth to its original condition.
Another approach to shear wave production utilizes a hydraulically-driven vibrator apparatus which rests upon the earth's surface and oscillates horizontally. This horizontal "shaking" motion is effective in producing horizontally polarized shear (SH) waves. However, such vibrator-produced SH waves are generated at the cost and limitations of providing a very expensive seismic source which will not produce compression waves when they are desired. Therefore, if is desired to carry out a seismic survey along a particular survey line employing both P waves and SH waves to obtain data resulting from both kinds of waves, it is necessary to utilize another seismic energy surface source in addition to such a horizontal vibrator. The use of such a second surface source to obtain P wave data entails a second operating crew with attendant costs in time and labor. If the second source and crew are not immediately available, then time will inevitably elapse before the second source is operated along that same survey line. It turns out that the characteristics of the uppermost tens of feet of the earth, that top surface layer being called the "weathering layer", change from day-to-day as a result of weather action. Consequently, when a significant time period has elapsed between the two crews' operations, such as a few days during which a storm has occurred, the weathering layer will have changed. Thus, the P wave data acquired by the second crew will not be directly correlatable with the SH wave data acquired by the first crew, since the weathering layer of the earth has changed in the intervening time period.
It is an object of the present invention to provide a seismic energy surface source capable of generating SH shear waves in the earth without regard to the local variability of the surface slope. Other objects are to provide such apparatus wherein the direction of initial thrust in generating SH waves can be easily controlled and wherein the same apparatus may be employed to produce either compression (P) or shear (SH) waves or both, and the relative proportions of P waves and SH waves to be generated can be varied easily and also wherein left-handed and right-handed SH waves can be generated. The manner in which the foregoing objects are achieved will become understood from the following description and from the appended claims.